CloudSat: On-Orbit Improvisation and the Decommissioning of a One-Wheel Extended-Mission Spacecraft

CloudSat, which was launched in 2006, has undergone several changes in its 17-year lifetime. Hardware anomalies have forced operational reconfiguration of the vehicle to continue its science mission past its original 22-month mission.In 2011, a battery anomaly necessitated developing the DO-Op (Daylight Only Operations) operational mode which demonstrated the flexibility of our spacecraft design and the ease of using tables to radically change the vehicle’s CONOPs without software modifications. In 2018 after a wheel loss, the program developed and utilized a method to perform burns using only thrusters to control the vehicle’s attitude to exit the A-Train, and later rejoin the C-Train with CALIPSO.Following the loss of another reaction wheel in July 2020, CloudSat began operations in December 2021 on one of its two remaining reaction wheels in its new ACT-TWO (Attitude Control Through Two Wheel Operations) mode of operations. This new method of control allows the vehicle to maintain payload pointing relative to nadir, maintain sufficient star tracker availability, maintain a positive power balance and a good thermal state for the observatory using one reaction wheel for attitude control. This new phase has been labeled ACT-TWO DO-OP since the spacecraft is still required to shed loads in eclipse to satisfy battery constraints. Science collection has continued in ACT-TWO DO-Op since December 2021 with regular orbit adjusts allowing it to maintain formation with its sister mission, CALIPSO. However, as both spacecraft have continued to age and with CALIPSO ceasing science operations in the summer of 2023, CloudSat will begin decommissioning activities in early 2024. A series of orbit lower maneuvers over several weeks will eventually leave CloudSat in its graveyard orbit. Decommissioning and passivation on any spacecraft program will require a great deal of planning and coordination among team members, even when the spacecraft has a mostly intact GNC sensor and actuator suite available. However, decommissioning CloudSat, with its myriad attitude and power constraints, requires additional thought and planning. Contingencies must be made for the loss of the reaction wheel that CloudSat is currently using for attitude control and making decisions on whether to transition to the other operational reaction wheel or using our thrusters-only mode to ensure the completion of the decommissioning timeline. This paper will focus on the operational planning required for this challenging activity, including contingency considerations. On-orbit data and lessons learned will also be discussed.